1. Field of the Invention
The present invention relates to an image pickup apparatus including pixels each having a photoelectric conversion unit therein and an analog-to-digital conversion unit.
2. Description of the Related Art
An image pickup apparatus including a pixel unit having pixels arranged in a matrix, each of the pixels performing photoelectric conversion and outputting a signal based on the incident light, and column parallel analog-to-digital conversion units (hereinafter referred to as “column ADCs”) each provided for one of the pixel columns has been developed. The column ADC provided for each of the columns performs analog-to-digital conversion (hereinafter referred to as “AD conversion”) in which an analog signal output from a pixel (hereinafter referred to as a “pixel signal”) is converted into a digital signal.
An image pickup apparatus described in Japanese Patent Laid-Open No. 2011-217206 includes a first pixel that performs photoelectric conversion on environment light and a second pixel that performs photoelectric conversion on light including environment light and reflected light. The image pickup apparatus further includes first and second comparators provided for the first and second pixels, respectively, a difference circuit connected to the first and second comparators, and a counter that counts the number of clock pulses based on a difference signal output from the difference circuit and generates a count signal. Hereinafter, such an image pickup apparatus is referred to as a “first image pickup apparatus”. In addition to the first image pickup apparatus, Japanese Patent Laid-Open No. 2011-217206 describes a second image pickup apparatus that has a third comparator in addition to the configuration of the first image pickup apparatus. The difference circuit of each of the first image pickup apparatus and the second image pickup apparatus outputs a difference signal to the counter from when the signal value of a comparison result signal of one of the first and second comparators varies to when the signal value of a comparison result signal of the other comparator varies. In the first image pickup apparatus, before the comparison operations are performed by the first comparator and the second comparator, different potentials are input to the two comparators. In this manner, in the first image pickup apparatus, a point in time at which the comparison result signal of the second comparator varies lags behind a point in time at which the comparison result signal of the first comparator varies at all times. Thus, a digital signal based on the difference between the analog signals output from the first and second pixels can be obtained regardless of a greater-lesser relationship between the analog signals output from the first and second pixels. In addition, in the second image pickup apparatus, by using the third comparator for sign detection, it can be determined which one of the analog signals output from the first and second pixels is larger.
Japanese Patent Laid-Open No. 2010-62764 describes an image pickup apparatus that outputs comparison result signals of a plurality of comparators each comparing a signal output from a different pixel with a reference signal. The image pickup apparatus includes a counter that switches between up-counting and down-counting based on the order in which the signal values of the comparison result signals of the plurality of comparators vary. In addition, during a period of time during which the signal values of the comparison result signals differ from one another, the image pickup apparatus causes the counter to operate to count the number of clock pulses. In this manner, a digital signal can be obtained based on the differences among the signals output from a plurality of pixels.
Japanese Patent Laid-Open No. 2010-28781 describes an image pickup apparatus that outputs comparison result signals of a plurality of comparators each comparing a signal output from a different pixel with a reference signal. If the signal values are the same, the count of the number of clock pulses per unit time is made double the count of the number of clock pulses per unit time when the signal values of the comparison result signals of the plurality of comparators differ from one another. In this manner, a digital signal can be obtained based on the sum of the signals output from a plurality of pixels 100.
Japanese Patent Laid-Open No. 2001-83407 describes an image pickup apparatus that includes a plurality of photoelectric conversion units each provided for one of microlenses two-dimensionally arranged in a microlens array and that performs focus detection of a phase difference detection scheme. In addition, the following three signals are output: a signal based on a signal carrier of a first photoelectric conversion unit of the photoelectric conversion units, a signal based on a signal carrier of a second photoelectric conversion unit of the photoelectric conversion units that differs from the first photoelectric conversion unit, and a signal based on the sum of the signal carriers of the first and second photoelectric conversion units.